Urban electric stations handling large DC currents, such as major substations of railway system, are required to have active magnetic shield for preventing emanation of excessively strong magnetic field into environment due to large DC currents flowing therein. On the other hand, houses located close to electric railway tracks or feeders supplying currents thereto are required to have passive magnetic shield for preventing outdoor magnetic noise from entering indoors, so as to maintain the inside of the houses clean. The outdoor or environmental magnetic noise is due to currents of the railway tracks or feeders. In recent years, various machines using strong magnetic field are installed indoors, for instance MRI (Magnetic Resonance Imaging) equipment in medical facilities within office buildings. In case of indoor machines with strong magnetic field, both active and passive shields are simultaneously required. The reason for it is in that outdoor persons and equipment must be protected from undesirable effects of the strong indoor magnetic field, such as those of MRI, and at the same time, the indoor machines themselves must be protected from environmental magnetic noise so as to ensure their proper performance.
With a typical conventional method for active magnetic shield, a space containing a magnetic flux generating source to be shielded is tightly enclosed by plates of magnetic material with a high permeability μ (hereinafter, magnetic material with a high permeability μ is referred to as simply “magnetic material”) so as to confine the magnetic flux therein. For instance, Patent Literature No. 1, which concerns a railway substation feeding DC power through feeding cables and return cables, discloses a magnetic shielding method by installing certain parts of the feeding cables and return cables side by side, so as to cancel the magnetic fluxes due to currents in said certain parts of the feeding and return cables with each other, while enclosing a required portion of the remaining parts of the cables by a duct made of magnetic material plates so as to suppress the magnetic flux density outside the duct due to the currents in said portion of the cables. In case of medical facilities using an MRI apparatus, undesirable effects of the MRI apparatus on the surrounding environment, such as nearby persons carrying heart pulse pace makers and nearby medical instruments, must be prevented. To this end, magnetic shielding is provided to a room housing MRI apparatus (MRI room), for instance by applying magnetic material plates to a portion or entire span of the sidewalls, ceiling and floor so as to maintain the magnetic flux density outside the MRI room at 0.5 mT or below.
In case of conventional passive magnetic shield, it is also typical to enclose tightly a space to be shielded with magnetic material plates so as to prevent outside magnetic flux from entering the space. For instance, a certain room in office building where office automation (OA) instruments are installed (OA room) has its slabs and sidewalls covered by magnetic material plates, so that magnetic noise or magnetic flux generated outside the OA room, indoors or outdoors, is prevented from entering into the OA room. In principle, the OA room may be shielded at six surfaces, four sidewalls and ceiling and floor. If, however, shielding against downstairs power source alone is necessary, a limited degree of protection can be provided by applying magnetic material plates only to selected floor areas, such as OA room floor. Patent Literatures No. 2 and No. 3 disclose techniques for improving the effectiveness of magnetic shield by using multiple layers of magnetic material plates or by filling joints of magnetic material plates with fillers made of specific magnetic material. Patent Literature No. 4 discloses a method of preventing magnetic flux leakage from openings of magnetic shield room by forming a closed magnetic path along the edge of such opening by using electromagnetic steel sheets.
[Patent Literature No. 1]
Laid-open Japanese Patent Publication No. 2001-231161
[Patent Literature No. 2]
Laid-open Japanese Patent Publication No. 1993-327263
[Patent Literature No. 3]
Laid-open Japanese Patent Publication No. 1995-273484
[Patent Literature No. 4]
Laid-open Japanese Patent Publication No. 1996-264350
[Patent Literature No. 5]
Laid-open Japanese Patent Publication No. 1996-288688
[Patent Literature No. 6]
Laid-open Japanese Patent Publication No. 2002-164686